A termination circuit for a two-wire transmission line has a function of matching a terminal impedance to decrease a reflected signal which is produced by a signal supplied from the two-wire transmission line to a four-wire transmission line, and is transmitted back thereto.
One type of a conventional termination circuit for a two-wire transmission line comprises a low-pass filter of an active filter, an analog to digital converter, a digital to analog converter, a terminal impedance resultant filter of a digital filter, and a feedback line having a feedback amount of "1".
Here, if it is assumed that a line impedance of a two-wire transmission line is Zs, and a transfer function defined by a cascade connection of the low-pass filter, the analog to digital converter, the terminal impedance resultant filter, and the digital to analog converter is Hz, the best result is obtained in a return-loss characteristic indicating a degree of suppressing the aforementioned reflected signal, when the following equation (1) is met. EQU Hz=-Ro/Zs (1)
where Ro is a reference resistance.
Another type of a conventional termination circuit for a two-wire transmission line comprises a low-pass filter of an active filter, a terminal impedance resultant filter of a switched capacitor filter, and a feedback line having a feedback amount of "1".
Here, if it is assumed that a line impedance of a two-wire transmission line is Zs, and a transfer function defined by a cascade connection of the low-pass filter and the terminal impedance resultant filter is Hz, the best return-loss characteristic is obtained, when the aforementioned equation (1) is met. Such a termination circuit is applied to, for instance, a switch board for a communication between two and four-wire telephone networks.
However, the former and latter termination circuits for a two-wire transmission line has a disadvantage, in that operation becomes instable in a high frequency range, in which a phase of an output signal at the low-pass filter is rotated by 180.degree.. In more detail, the output signal phase begins to be rotated at a frequency more than 80 kHz. Then, the phase is rotated around 100 to 200 kHz by 90.degree., and at a frequency more than 400 to 500 kHz by 180.degree.. As a result, a negative feedback which is customarily fedback through the terminal impedance resultant filter in accordance with the output signal of the low-pass filter is turned to be a positive feedback in opposition to operation inherent to the circuit.